Field of the Invention
The present invention relates to a unidirectional microphone unit, and especially relates to a unidirectional microphone unit that allows to obtain stable frequency characteristics.
Description of the Related Art
Conventionally, a unidirectional microphone has openings (sound holes 51 and 52) in the front and rear area of a microphone unit 50 to collect sound waves, as illustrated in FIG. 6. Accordingly, a front acoustic terminal 61 occurs on the front side of the unit, and a rear acoustic terminal 62 occurs on the rear side of the unit. The acoustic terminal refers to a position of air that effectively provides a sound pressure to the microphone unit 50. In other words, the acoustic terminal is a central position of air simultaneously moving with a diaphragm included in the microphone unit. In a case of the unidirectional microphone unit, the acoustic terminals occur in a front portion and a rear portion of the diaphragm, as described above.
A fixed distance is provided between the front acoustic terminal 61 and the rear acoustic terminal 62, and this fixed distance is called a distance between acoustic terminals. Note that, in FIG. 6, the solid line connecting the acoustic terminals 61 and 62 is a distance between acoustic terminals for high-frequency range, and the broken line is a distance between acoustic terminals for low-frequency range. Conventionally, the distance between acoustic terminals for high-frequency range and the low-frequency range coincide with each other approximately.
The sound waves entering the microphone respectively enter the front acoustic terminal 61 and the rear acoustic terminal 62 with a time difference. The sound waves having entered from the front acoustic terminal and the rear acoustic terminal, respectively, pass through different routes, and overlap with each other at a front side and a rear side of the diaphragm. Through this wave synthesis, only the sound wave arriving from the rear acoustic terminal is cancelled, and unidirectionality is realized.
By the way, the distance between acoustic terminals is determined by a microphone shape, and which sometimes causes to occur acoustical inconvenience. For example, problems may occur such that an output is decreased in a specific frequency due to relationship between the distance of acoustic terminals and a wavelength, and howling is more likely to occur because output levels in a front direction (0° direction) and a backward direction (180° direction) become the same.
Further, dips and peaks occur in frequency response as the sound waves are mutually cancelled or overlap with each other due to the relationship between the distance between acoustic terminals and the wavelength. The occurrence of the dips and peaks becomes a cause failing to maintain the frequency characteristics constant.
To address the problems, in a microphone disclosed in JP 2013-223057 A, the shape of a housing is changed to adjust a distance between acoustic terminals. A unidirectional condenser microphone disclosed in JP 2013-223057 A includes a directivity variable member formed into a hollow tubular shape arranged to contact closely to an outer periphery of a cylindrical microphone case. The directivity variable member is formed of a porous sintered material. Selection of a mode to cover a rear acoustic terminal with the directivity variable member, and a mode to release the rear acoustic terminal and shift a front acoustic terminal to the front of the microphone case enables fine adjustment of directivity.
However, if the directivity variable member is attached around the microphone case, like the microphone disclosed in JP 2013-223057 A, an external size of the microphone becomes large, and the distance between acoustic terminals becomes long. Therefore, there is a problem of a decrease in drive force in a high-frequency range where the wavelengths are short.
To address the problem, conventionally, adjustment of an distance between acoustic terminals and a resonance has been performed with a metal-made housing 70 having notches 71 communicating with rear sound holes 72, as illustrated in FIG. 7A. Alternatively, holes 81 communicating with rear sound holes 82 are formed in a metal housing 80 by drilling, as illustrated in FIG. 7B, and a wavelength to pass is adjusted by setting of a hole diameter. However, these conventional microphones have a problem of high manufacturing cost.